Digital Instrumentation, by A. J. Bowens and published by McGraw Hill, describes at pages 223 through 226 circuitry which develops an indication of the amplitude of a signal being measured from the duty cycle of a series of pulses produced in response to the difference in amplitudes between the signal being measured and a signal having an amplitude dependent upon the duty cycle of the series of pulses. This amplitude measuring technique is identified as "delta pulse modulation."
The "delta-pulse modulation" circuitry described in Digital Instrumentation suffers from a serious shortcoming. In this circuitry, the signal representative of the duty cycle of the pulses and against which the amplitude of the input signal is compared is derived from a series of pulses which can have as many transitions in the generation of the series of pulses as there are elements of resolution. This is due to the fact that each clock signal from the clock oscillator which defines the resolution of the circuitry has the potential for causing the development of a pulse which contributes to the development of the indication of the amplitude of the signal being measured. As a result, errors due to imbalance and asymmetry in the rise and fall times of the pulse duty cycle are introduced for each such pulse. The cumulative effect can produce erroneous indications of the amplitude of the signal being measured.